1. Field of Invention
This invention relates to methods and apparatus for welding and is particularly directed to improved methods and apparatus for welding high strength steels and the like.
2. Prior Art
Welding has long been a preferred method for joining metal pieces because the resulting bond is often stronger than the surrounding material and, hence, produces a reliable and long lasting joint. However, progress in science and industry constantly demands new metals which will be stronger and more durable and which will withstand increasingly more hostile environments. At the same time, the metallurgical scientists continue to produce such new metals and new technologies for welding these new metals. One example of the challenges of science and industry is the recent efforts of the world's navies to build submarines which can withstand the severe pressure of submerging deeper than any previous submarines and which can survive impact loading, such as depth charge attacks, icebergs, torpedoes, reefs collisions, etc. at these extreme depths and pressures.
As is well known, welded joints are designed primarily on the basis of the mechanical properties and safety required of the weldment under the service conditions to be imposed on it and the manner in which the service stresses will be applied must always be considered. Also, regardless of their service requirements, the joints must be designed for economy of fabrication. The most widely used method of welding such structures is gas metal arc welding (GMAW). This is an electric arc welding process which produces coalescence of metals by heating them with an arc established between a continuous filler metal (consumable) electrode and the work. In Europe, gas metal arc welding is known as Metal Inert Gas (MIG) welding, when the shielding gas is inert, known as Metal Active Gas (MAG) welding when the shielding gas contains reactive gases, such as O2 or CO.sub.2 or mixtures of inert and reactive gases. Other high efficiency welding processes, with higher contamination levels, which are commonly used for such welding include submerged arc welding, shielded arc welding, flux cored arc welding and electo-slag welding. Shielding of the arc and molten weld pool is obtained either by an externally supplied gas, which may be inert, reactive or a mixture of inert and reactive gases, or by use of a suitable flux material, which also may contain contaminating materials. It is found that inert gases yield the highest quality weldments, while mixtures of inert and reactive gases produce better weldments than reactive gases alone. These processes have the advantages of being relatively fast and inexpensive. Unfortunately, as service requirements become increasingly severe, there is increasing demand for use of alloy steels. However, these steels may become embrittled during the welding, as a result of the addition of CO.sub.2 or oxygen needed to stabilize the arc. Most shielding gases and flux materials contain contaminents, such as O.sub.2, CO.sub.2, H.sub.2 or compounds containing these substances, which will embrittle the weld. While pure inert gases, such as argon, would not embrittle the weld, efficient metal transfer is not obtained and it is necessary to add at least some reactive gas, such as CO.sub.2, to the GMAW gas shield or flux material. Gas tungsten arc welding (GTAW) is a process wherein coalescence of metals is produced by heating them with an arc between a tungsten (nonconsumable) electrode and the work and usually uses inert shielding gases, such as helium or argon. Shielding of the arc and weld zone is obtained by employing a blanket of a high purity inert gas, such as argon, which is fed through the electrode holder. Because it relies on high purity gas shielding, the GTAW process has been found to be effective in joining high strength metals and alloys and produces a joint which is extremely tough and strong. However, this process is considerably slower and less efficient than GMAW and, because of the costs of the inert shielding gas and the tungsten electrodes, GTAW is considerably more expensive than GMAW. For these reasons, GTAW is generally not economically competitive with GMAW for welding heavy gages of metal. Producing modern submarines requires welding of heavy gages of high strength metal alloys, which means either employing GTAW to produce a weld of maximum strength, at the sacrifice of time and cost, or to use GMAW to save time and money but produce a joint having reduced strength. It has also been known to overlay a welded joint with a layer of low heat transfer weld to provide a cosmetic finish. However, when this is done, the cosmetic overlay is usually a very thin layer which is applied only for appearance and is not applied continuously along the entire weld joint nor is it intended to provide structural advantages. Thus, none of the prior art welding techniques have been entirely satisfactory.